1. Field of the Invention
The present invention relates to an input device of a mobile electronic apparatus such as a portable telephone, specifically to an input device capable of inputting the direction and strength of a pressing force when a key top (operating member) is pressed.
2. Description of the Related Art
The input device of a conventional portable telephone is made up with a printed circuit board 81, metal contacts 83, a contact sheet 84, and a key top 85, as shown in FIG. 9 and FIG. 10.
The printed circuit board 81 has plural pairs of fixed contacts 86a, 86b printed on the surface thereof, as shown in FIG. 9.
FIG. 10 illustrates the switch structure to put a pair of the fixed contacts 86a, 86b into a conductive state, and the other fixed contacts have the same structure. As shown in FIG. 10, the metal contact 83 overlies the fixed contacts 86a, 86b mounted on the printed circuit board 81 so as to face each other, and the contact sheet 84 is adhered on the printed circuit board 81 with the metal contact 83 put in between.
The metal contact 83 is made of a domed thin metal having a structure capable of elastic deformation, and the circumference thereof is in contact with the fixed contact 86b. 
And, the key top 85 overlies the contact sheet 84 vertically movably to face the metal contact 83, and has a downward convex projection formed to face the center of the metal contact 83.
Now, in this state, pressing the key top 85 downward deforms the metal contact 83 to bring both the pair of fixed contacts 86a, 86b into contact, whereby the pair of fixed contacts 86a, 86b are made conductive, and a key input detection circuit (not illustrated) detects that the fixed contacts 86a, 86b are conductive, thereby discriminating a key input.
When a pressing load to the key top 85 is released, the key top 85 returns to the original position by the restoring force of the metal contact 83, and the fixed contacts 86a, 86b are made non-conductive.
In the same manner as the fixed contacts 86a, 86b, a set of fixed contacts 88, 89, 90, 91 that allow four inputs of right, left, up, and down directions is printed on the upper front of the printed circuit board 81, as shown in FIG. 9.
When the input operation of the corresponding direction is made by a pair of the fixed contacts 88, 89, 90, 91 being made conductive as mentioned above, the portable electronic apparatus shifts the cursor according to the operated direction, and scrolls the characters and images displayed.
Recent developments have implemented some input devices detect not only the on-off digital inputs of up, down, right, and left directions but also detect the strength of force (analog value) during the operation.
In the input device as shown in FIG. 11, four resistors 92, 93, 94, 95 are printed on the printed circuit board 81, instead of the fixed contacts 88, 89, 90, 91 for four directional inputs in FIG. 9.
FIG. 12 is a sectional view that illustrates the upper structure of the resistor 92 of these resistors, and the other resistors 93, 94, 95 have the same structure as well.
As shown in FIG. 12, the resistor 92 is printed on the printed circuit board 81, and a downward convex conductive rubber 96 is located to overlie the resistor 92.
The conductive rubber 96 is fixed to the key top 85 by way of an elastic body 97. The key top 85 is energized upward by an elastic body not illustrated, and the resistor 92 faces the conductive rubber 96 with a slight spacing.
In this state, when the key top 85 is pressed down, the conductive rubber 96 is lowered, the conductive rubber 96 comes in contact with the resistor 92, the contact area made by the conductive rubber 96 and the resistor 92 varies according to the pressing force, and the resistance across the resistor 92 varies.
As the pressing force to the key top 85 becomes stronger, the contact area of the conductive rubber 96 and the resistor 92 becomes larger; and as the contact area becomes larger, the variance (decrease) of the resistance across the resistor 92 becomes greater. Therefore, it is possible to calculate the force applied to the key top 85 by detecting the variance of the resistance.
When the load to the key top 85 is released, the key top 85 returns to the original position by the restoring force of the elastic body not illustrated, and the resistor 92 is made to face the conductive rubber 96 with a slight spacing.
The circuit to detect the resistance of the resistor 92 is configured with the four resistors 92, 93, 94, 95, and a controller (CPU) 98, as shown in FIG. 13.
The controller 98 has analog voltage input terminals A/D1, A/D2 that input analog voltages, and plural input/output terminals not illustrated, etc.
One end of the resistor 92 is connected to one end of the resistor 93, and the resistors 94 and 95 are connected in the same manner. The other ends of the resistor 92 and 94 are connected to a power supply Vcc, and the other ends of the resistor 93 and 95 are connected to the ground.
The node of the resistors 92 and 93 is connected to the analog voltage input terminal A/D1, and the node of the resistors 94 and 95 is connected to the analog voltage input terminal A/D2.
And, the controller 98 is designed to monitor the analog voltages to be inputted to the analog voltage input terminals A/D1 and A/D2.
In the initial state that the resistors 92, 93, 94, 95 and the conductive rubber 96 face each other with a slight spacing, the resistances of the resistors 92, 93, 94, 95 are equal, and both the two analog voltage input terminals A/D1, A/D2 input the voltage of Vcc/2 accordingly.
In this state, when the key top 85 is pressed to bring the conductive rubber 96 into contact with the resistor 92(X+), for example, the resistance across the resistor 92 decreases; and accordingly, the analog voltage input terminal A/D1 inputs a voltage higher than Vcc/2, and the analog voltage input terminal A/D2 inputs the voltage of Vcc/2, which is not varied.
Here, since the voltage inputted to the analog voltage input terminal A/D1 has become higher than Vcc/2, the controller 98 detects that the conductive rubber 96 comes into contact with the resistor 92(X+), calculates the variance of the resistance of the resistor 92 from the analog voltage, calculates the strength of the force that has pressed the key top 85 from the variance of the resistance, and outputs the result from the output terminal (not illustrated).
In the portable electronic apparatus, the direction of shifting the cursor and the direction of scrolling the images are controlled from the direction thus outputted, and the speed of shifting the cursor and the speed of scrolling are controlled from the strength of force thus outputted.
However, in this type of input device having the fixed contacts 86a, 86b printed on the printed circuit board 81, there occur wear and smear on the fixed contacts 86a, 86b, which lowers the reliability and shortens the life of the contacts, thus presenting a problem to be solved.
If the number of key switches is increased to make the input device multi-functional, the fixed contacts 86a, 86b will have to be printed on the printed circuit board 81 by the number of switches to be increased; and since the wiring space is required for the number of switches increased, to miniaturize the printed circuit board 81 has been an obstacle to make the input device multi-functional.
Further, since the defects of the contact sheet 84 cannot be discovered before the contact sheet 84 is adhered on the printed circuit board 81, the replacement of the defective contact sheet 84 has required trouble of peeling it off the printed circuit board 81, and so on.
Further, since the conductive rubber 96 is put into contact with the resistor 92 to produce the variance of the resistance, and in addition to the short life of contacts of the conductive rubber 96 itself due to abrasion, and further since the conductive rubber 96 is directly pressed to the resistor 92 made of carbon, for example, the life of the conductive rubber 96 by abrasion is further shortened accordingly, which is a problem.
Furthermore, in the circuit to detect the resistances of the resistors, since the controller 98 has to be monitoring the analog voltages inputted to the analog voltage input terminals A/D1 and A/D2, the power consumption increases only to shorten the continuous operational time, which is a problem.